1. Field of the Invention
This invention relates to a method of braking force distribution and a braking force control system for a vehicle.
2. Description of the Related Art
There has been a problem, if braking force is applied to both of a front wheel brake and a rear wheel brake of a four-wheel automotive vehicle or the like, that a center of gravity of the vehicle shifts forward and a vertical load of the rear wheels becomes smaller, so that the rear wheels are likely to be locked.
To counter this problem, there has been proposed a braking force distribution method for providing proper distribution between braking force on the front wheels and braking force on the rear wheels, depending on load shifting due to load condition change or deceleration, whereby the brakes can stably work.
Such a method of braking force distribution has been usually realized by providing proportioning valves in brake pipes. Recently, a braking force distribution method employs control logic on a braking force control system for a vehicle, which is capable of anti-lock brake control.
For example, JP2003-118552A discloses a method of braking force distribution in which, if a value yielded by subtracting a rear wheel speed from a front wheel speed is more than a first predetermined value, brake hydraulic pressure on the rear wheels is reduced so as to reduce a braking ratio of braking force on the rear wheels so that on the front wheels; if the subtracted value is smaller than a second predetermined value that is smaller than the first predetermined value, the brake hydraulic pressure on the rear wheels is boosted so as to increase the braking ratio of the braking force on the rear wheels to that on the front wheels. In the method of the braking force distribution disclosed in JP2003-118552A, a target wheel speed of the rear wheels is an actual wheel speed of the front wheels, and the braking force on the rear wheels is controlled such that the actual wheel speed of the rear wheels follows the actual wheel speed of the front wheels. Accordingly it is possible to control distribution between the braking force on the rear wheel side and the braking force on the front wheel side. The wheel speed can be obtained by multiplying rotation speed (angular velocity) of the wheel detected by a wheel speed sensor provided on the wheel to a radius of the wheel (tire radius).
By the way, not to mention when the rear wheels are equipped with tires having different radius from those of the front wheels, if there occurs a difference between a radius of the front wheel tires and that of the rear wheel tires due to, for example, a difference in abrasion rate or air pressure, an inevitable difference occurs on rotation speed between the rear wheel and the front wheel regardless of an existence of the wheel lock. Hence, it is required to grasp an actual radius of each tire. Otherwise, there occurs a difference in wheel speed that is obtained by multiplying a tire radius by the rotation speed.
Even if each front and rear tire has the same radius, the rear wheel have a different turning radius from that of the front wheel, therefore, there occurs an inevitable difference in rotation speed between the rear wheel and the front wheel, as well.
Specifically, a value yielded by subtracting the rear wheel speed from the front wheel speed includes an error due to difference in tire radius between the rear wheel and the front wheel. If the braking force on the rear wheel is boosted or reduced based on a value including such an error, electromagnetic valves provided in the brake hydraulic pressure circuit excessively work, resulting in producing unpleasant noises which may not only give passengers unpleasant feelings but also hinder driver's braking feelings.
In the light of the above problems, there has been requested a method of braking force distribution for properly controlling distribution between braking force on the front wheels and braking force on the rear wheels, in which, even if an inevitable difference in wheel speed exists between the rear wheels and the front wheels, it is possible to properly control distribution between the braking force on the rear wheel side and the braking force on the front wheel side. There has also been requested a braking force control system for a vehicle realizing such a method for braking force distribution.